Only allow STREAM_MUSIC as driving stream for AVRCP

The BT AVRCP service is storing the last volume of STREAM_MUSIC when
something changes. This is inconsistent with the audio framework which
now allows to have STREAM_VOICE_CALL drive the absolute volume too.

As a quick fix, enforcing on the audio fwk side to be consistent with
the AVRCP service and report only STREAM_MUSIC as the driving stream for
AVRCP.

Test: bug repro with active STREAM_VC when AVRCP active
Bug: 379917442
Bug: 382366084
Flag: EXEMPT bugfix
Change-Id: I4e87edfc938d34c341b0012736e1af2affaa2746